The proposed work is designed to elucidate the neural and reflex control of the heart. A quantitative analysis of spontaneous cardiac autonomic neural activity will be attempted by precise assessment of several critical aspects of cardiac performance which are differentially influenced by sympathetic and vagal activity. The role of the baroreceptor reflexes in the beat-by-beat regulation of heart rate will be assessed by studying the influence of changing patterns of carotid sinus nerve stimulation. Also, the effects of the precise timing of vagal activity on A-V nodal conduction will be assessed. The role of the cardiac autonomic nerves in certain cardiac arrhythmias will also be studied, specifically in the A-V nodal Wenckebach phenomenon and in concealed bigeminy. Control theory will be applied to analyze the Wenckebach phenomenon as a positive feedback mechanism.